For some IR sensor applications, it is generally not possible to meet two critical performance requirements with the same system design configuration: very fast cooldown time (seconds to reach Sensor operating temperature) and long system operational run times (enabling the system to operate for thousands of hours without maintenance or service). In some cases, the ability to abort a mission and re-use the device at a later date can be a desirable feature and adds operational flexibility. This is not possible with current technology.
The requirements for achieving very quick cooldown time to operating temperature and maintaining long operational run times are considered mutually exclusive for applications where weight, size and power are a premium. Applications such as a seeker on a missile or a surveillance sensor, generally need to be small, lightweight, portable and adaptable. So there is generally a trade-off between quick cooldown time and operational run time because of size and weight constrains.
Cryocoolers designed for applications requiring very fast turn on times are generally based on the Joule Thomson (J-T) effect because of the very high rates of cooling achievable with this cooling cycle. As a result, missile applications typically use conventional J-T cooling approaches because fast cooldown times are crucial to the program. However, J-T type coolers suffer from relatively short run times because of the size, weight and power penalty associated with running these coolers for long periods of time. J-T cryostats can be made very small, lightweight and compact but lack operational run time. To achieve very long operational run times, these coolers require either large reservoir volumes of very high pressure gasses or very large compressors to supply very high pressure gasses. Both solutions add to the size weight and power to the device.
Applications which require long run times are met with other types of coolers. These cryocoolers are generally Stirling and pulse-tube based and meet the long continuous operation requirements. They are small in size, light weight and have very high efficiencies. However, they cannot match the high cooling rates of J-T coolers and therefore suffer from longer cooldown times when compared to J-T cryostats. Making such Stirling and pulse-tube cryocooler systems have shorter cool-down times also adds bulk and power. Either solution results in very large, heavy and bulky systems.
Furthermore, the future trend in sensor technology is to employ larger sensors packaged in smaller spaces, light weight and requiring very low power to operate. In addition, these sensors have to be cooled reliably to operating temperatures and meet a host of other specific performance requirements such as the time the sensor requires to get to its operating temperature, the efficiency of the process, minimum input power to maintain temperature, the size and weight of the system, the operating time for a mission and the operating life of a system. Currently, the type of coolers used to achieve these performance requirements are large and are not easily adaptable to meeting diverse requirements because they were designed purely and solely for achieving cryogenic temperatures.